1. Field of the Invention
The present invention relates generally to a linear tracking pickup apparatus which is used for reproduction of recorded sound or video signals from a phonograph or video disc and, more particularly, is directed to a device for driving a tone arm in such linear tracking pickup apparatus.
2. Brief Description of the Prior Art
In order to completely eliminate lateral tracking error and inside forces in a linear pickup device, there has been proposed and put into practice a so-called linear tracking pickup device.
In some record playing devices of the linear tracking type, a screw feeding mechanism and a wire are used for linearly driving the tone arm. However, since the manufacturing accuracy of the screw feeding mechanism and the wire are not very high, the tone arm is not smoothly moved. On the other hand, a record playing device has been proposed where the tone arm is driven along guide rails by a linear drive motor. However, when electric power is removed from the linear drive motor, the tone arm can be freely moved, since there is no force to hold the arm. Generally, the tone arm is supported by a carriage, and the carriage and tone arm are moved together and constitute a single assembly. The weight of such assembly, however, is considerably large so that when the record playing device is inclined with respect to the horizontal plane, the tone arm and carriage move due to the force of gravity. Further, when the record playing device is supported by flexible support members on a base, the record playing device may be moved to an inclined position as a result of movement of the tone arm and carriage assembly.
Thus, during reproduction of sound from the disc, when the tone arm is unexpectedly moved by vibration of the drive motor or by vibration from an external source, such as the floor, the reproduced output signal is correspondingly modulated, and cross-talk between the right and left channel signals is increased in a stereo reproduction. If the frequency of the vibration is near, or coincides with, the resonant frequency of the tone arm, such cross-talk phenomena is greatly increased.
In the record playing device of the linear tracking type, the tone arm is also moved by the grooves of the record disc, and is angularly displaced in the direction normal to each groove of the disc. Whenever the angular displacement of the arm reaches a predetermined value, the tone arm is driven linearly so as to cancel such angular displacement. Thus, the rotation of the tone arm and linear movement thereof are alternately repeated. The tone arm thus moves linearly, swinging about its axis. However, when the swing movement is too intermittent, signals cannot be reproduced from the disc with good fidelity. It is therefore desirable that the repetition of the rotation-linear movement of the tone arm occurs at very fine and frequent intervals so that the tone arm is smoothly moved across the surface of the record disc.
Further, when a counter-weight is used for adjusting the transducer pressure on the disc, the rotational axis of the tone arm does not coincide with the gravitational center of the arm so that a rotational movement is imparted to the arm during linear movement thereof. As a result, an angular displacement is added to the proper angular displacement thereof whereby linear movement of the arm becomes unstable.
Further, in the record playing device of the linear tracking type, the tone arm is rotatably supported on a carriage which is linearly moved along guide rails. With such arrangement, the rotation of the arm relative to the carriage should be braked, for example, by a motional feedback control, to smoothly and linearly move the arm. Therefore, a rotation detector and a drive motor are provided for the motional feedback control. However, when such elements are mounted on the carriage, a relatively large space is required, in the direction of the length of the rotational axis of the arm, to house such elements, resulting in level adjustment of the tone arm being limited.